/Engine/Plugins/Runtime/nDisplay/Source/DisplayClusterConfiguration/Public/DisplayClusterConfigurationStrings.h
Always use 'none' for headless rendering.
namespace DisplayClusterConfigurationStrings { namespace config { namespace cluster { namespace render_sync { static constexpr const TCHAR * DisplayClusterConfigurationStrings::config::cluster::render_sync::HeadlessRenderingSyncPolicy = None; } } } }